Title: Forming planetary systems that contain only minor planets
Authors: Dimitri Veras, Shigeru Ida
First Author’s Affiliation: Centre for Exoplanets and Habitability, Centre for Space Domain Awareness, and Department of Physics, University of Warwick, Coventry CV4 7AL, UK
Status: Accepted for publication in MNRAS
How many Solar Systems are there in the Milky Way?
With the discovery of thousands of exoplanets, we have gained a better understanding of the diversity and complexity of planetary systems. This has led planetary scientists to ask the question: “what is the true fraction of planetary systems in the Milky Way?” Exoplanet detection techniques have generally been biased towards higher mass gas giants and rocky planets. Giant planets like Jupiter are found around 10% of stars. But if we include smaller planets like Super-Neptunes and Mini-Neptunes, the number goes up to tens of percent. And when we look at even smaller planets like Earth, we find them around more than half of the stars. So, what would happen if we could detect much smaller planets– perhaps those as big as Mercury and Pluto, or even smaller?
To simulate small rocky planetary systems, the authors have to make estimates for how much material is available in star-forming regions! The dust and gas that goes on to form stars will form the accretion disks around the stars that will go on to form the planets. Since it’s challenging to determine exact mass is available in star-forming regions, the authors of this paper instead assume that very low-mass disks of gas and dust exist and explore what happens to tiny planet seeds as they grow and evolve in these environments.
Several cases of simulations are considered:
- Fiducial case: This case simulates systems where all minor planets are under the masses 10-5, 10-4, 10-3, 10-2, 10-1, and 10 Earth masses, as shown in Fig. 1. All bodies in this case would be considered “rocky”, or “icy” where they are far enough from the host star.
- Dependence on distribution of dust and gas: By varying the steepness of dust and gas distributions (q and p, respectively in Fig. 2), it was found that minor planet formation is a stronger function of dust than of gas.
- Dependence on disk dispersal timescale: These simulations are run with two disk dissipation timescales: 0.32 and 32 Myrs, representative of dissipation timescales for the vast majority of stellar systems whose disks undergo gas depletion. The simulations show that if a disk lasts longer, it allows for the formation of higher-mass minor planets. However, the authors are most interested in if longer dissipation timescales impact the formation of minor planets and Earth-sized planets similarly. They find that longer dissipation timescales does not impact the formation of minor planets, but /is/ important for forming Earth-sized or larger planets!
- Varying initial dust mass: Over 10000 simulations were run to evaluate the formation of these minor planets as a function and they found a total dust mass of 10-100 Earth masses is the most efficient in generating sub-terrestrial systems (i.e. planetary systems with only sub-Earth-mass minor planets).
Post Formation Mechanics
After the disk is dissipated, the minor planets in the system don’t change much. Without larger planets like Earth or Jupiter to disturb them, these tiny planets mostly stay in stable orbits. They’re spread too far apart to interact with each other significantly, so their orbits remain relatively unchanged for billions of years.
When a star expands and becomes a giant during its late life stages, the small planets around it face two possible fates, as illustrated in Fig 3. Those close to the star might get swallowed up, but if their orbits move outward quickly enough as the star loses mass, they might escape this fate. The planets farther away won’t be swallowed but will still face harsh conditions like extreme radiation and melting. They won’t be ejected from the system, so they could survive in some form until the star becomes a white dwarf.
If the star eventually becomes a white dwarf, these small planets can survive for even longer, although they might break up or partially vaporize due to the high stellar radiation from a white dwarf. The remaining planetary material can pollute the surface of the white dwarf, but it is hard to predict how much material from the planetary system would survive this long!
Extrapolating to Observed Exoplanet Systems
To test the validity of their simulations, the authors extrapolate their favored model from those described above to observed exoplanet systems. It is found that the simulations aligns well with theoretical predictions and known aspects of modern exoplanet populations. One key finding highlighted in this paper is that Super-Earths and Mini-Neptunes are common classes of exoplanets in our galaxy. Their result supports previous studies that have found these types of planets to be prevalent in our Milky Way galaxy.
Capturing Trends in Exoplanetary Systems
In addition to overall planetary system formation, the authors also investigate specific phenomena seen in actual exoplanetary systems. For example, they explore inward migration – a process where planets move closer towards their host star over time due to interactions with other objects or forces within the system. Their simulations also capture another interesting trend seen in many exoplanetary systems – the “Neptunian desert.” This refers to a lack of Neptune-sized planets at small orbital distances from their host stars. Their results suggest that this phenomenon can be explained by accretion processes during planetary formation.
Final Thoughts
Overall, this study (which seems to be the first in a possible series on this subject) suggests that minor planet-only systems may be more prevalent throughout our galaxy than previously thought. These findings not only contribute significantly to our understanding of planetary system formation but also offer insights into unresolved questions regarding planet-hosting white dwarfs in our galaxy. The authors also note that their simulations provide a possible explanation for the presence of minor planet-only systems under the right set of initial conditions including evolutionary timescales, dust mass budget, and host star mass .
Astrobite edited by Erica Sawczynec
Featured image credit: ESO/M. Kornmesser
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Interesting, but Pluto is NOT a minor planet, as the term “minor planet” is a synonym for asteroids and comets, objects not large enough or massive enough to be rounded by their own gravity.
The IAU definition for a minor planet is: Object in orbit around that our Sun that is exclusively classified as neither a planet nor a comet.
This includes asteroids, trans neptunian objects, and centaurs– pretty much anything thay doesnt fit the IAU definition of a planet. Well, anything except coments. Pluto is such an objecy which is neither a planet and not a comet. Dwarf planets can be categorized as minor planets, even though colloquially people use the term “minor planet” to describe asteroids.